I'd recommend creating an .epub in
Sigil first and converting it with Calibre or
Kindlegen to a .mobi file. (You can also convert your book to an .epub file with Calbre and then edit it with Sigil.)
Either way, you'll need to add the following guide item to the .opf file:
Code:
<guide>
<reference href="Text/Chapter1.xhtml" title="Chapter 1" type="text" />
</guide>
Of course, you'll need to change the file path so that it points to the start of your book.
See also this
web site for more information about guide items in general.